Mode of inhibition of DNA replication in neocarzinostatin-treated HeLa cells

Neocarzinostatin as a probe for DNA protection activity--molecular interaction with caffeine

Neocarzinostatin (NCS), a potent mutagen and carcinogen, consists of an enediyne prodrug and a protein carrier. It has a unique double role in that it intercalates into DNA and imposes radical-mediated damage after thiol activation. Here we employed NCS as a probe to examine the DNA-protection capability of caffeine, one of common dietary phytochemicals with potential cancer-chemopreventive activity. NCS at the nanomolar concentration range could induce significant single- and double-strand lesions in DNA, but up to 75 ± 5% of such lesions were found to be efficiently inhibited by caffeine. The percentage of inhibition was caffeine-concentration dependent, but was not sensitive to the DNA-lesion types. The well-characterized activation reactions of NCS allowed us to explore the effect of caffeine on the enediyne-generated radicals. Postactivation analyses by chromatographic and mass spectroscopic methods identified a caffeine-quenched enediyne-radical adduct, but the yield was too small to fully account for the large inhibition effect on DNA lesions. The affinity between NCS chromophore and DNA was characterized by a fluorescence-based kinetic method. The drug-DNA intercalation was hampered by caffeine, and the caffeine-induced increases in DNA-drug dissociation constant was caffeine-concentration dependent, suggesting importance of binding affinity in the protection mechanism. Caffeine has been shown to be both an effective free radical scavenger and an intercalation inhibitor. Our results demonstrated that caffeine ingeniously protected DNA against the enediyne-induced damages mainly by inhibiting DNA intercalation beforehand. The direct scavenging of the DNA-bound NCS free radicals by caffeine played only a minor role.

Hypersensitivity and reduced inhibition of DNA synthesis in ataxia telangiectasia lymphoblasts treated with low levels of neocarzinostatin

The effects of neocarzinostatin (NCS) on lymphoblastoid cell lines (LCLs) established from ataxia telangiectasia (A-T) were determined. A-T lymphoblasts were found to be hypersensitive to low levels of NCS as measured by cell growth and cell survival. On the other hand, A-T lymphoblasts failed to postpone DNA synthesis to the same degree as normal lymphoblasts following treatment with NCS. LCLs established from Nijmegen breakage syndrome (NBS) could be distinguished from ataxia and normal cell lines by their intermediate level of survival following exposure to NCS.